This month, polymer expert and technical blog author, Dr Charlie Geddes, goes back to the classroom when it comes to friction.
Friction is an important property for thermoplastics in bearings and gears, but it also has a part to play in assembly of plastic parts (snap-fit and interference-fit) and ejection during moulding.
When two surfaces rub together, the resisting force is the combination of two effects at the surfaces; an adhesion force, caused by attraction between the two surfaces at a molecular level, and a deforming force, related to the deformation of the nano-scale ‘peaks’ on the surfaces. Because these two mechanisms stem from different characteristics of thermoplastics, it is difficult to predict overall friction from first principles and values of the coefficient of friction have to be acquired from physical tests.
Despite what you learned at school, the coefficient of friction for thermoplastics is not entirely independent of load and velocity. You would be correct in thinking that temperature is another variable. For smooth surfaces and low speeds, the adhesion factor dominates, favouring non-polar materials such as PTFE and HDPE. At high speeds and rough surfaces, the deforming force is more important, favouring more rigid thermoplastics, either from high crystallinity (polyacetal) or with reinforcing fillers (glass or carbon fibre). Low levels of additives (PTFE, silicone and molybdenum disulphide) can significantly reduce friction.
Friction data for thermoplastics rubbing against steel are fairly readily available, but not for two thermoplastics in contact. In both cases data will be for a limited set of conditions, probably unrelated to the service condition in question.
Friction between components of the same thermoplastic can be considerably higher than for dissimilar materials, which is why different thermoplastics are used for alternate cogs in gear trains. If no friction data is available, selecting HDPE for the alternate gears is normally a safe choice for minimising friction…but wear is another story.